Metalorganic compounds

ABSTRACT

A process is provided for the production of metalorganic compounds by reacting a Grignard reagent with a Group II, Group III or Group V metal halide in a tertiary alkyl amine solvent to form a metalorganic adduct, isolating the adduct and dissociating the adduct to leave the metalorganic compound.

This application is the U.S. National Stage of International applicationSer. No. PCT/GB95/02087, filed Sept. 4, 1995, which derives its priorityfrom Great Britain Patent applications Ser. No. 9508702.9, filed Apr.28, 1995 and 9417707.8, filed Sept. 2, 1994.

DESCRIPTION

This invention concerns metalorganic compounds, especially metalorganiccompounds used in the growth of semiconductor layers by vapour phaseepitaxial techniques, such as by chemical beam epitaxy, MOVPE, or ALE.

Metalorganic precursors used in semiconductor growth are generallysynthesised by reacting a Grignard reagent such as an alkyl magnesiumhalide RMgX or an alkyl lithium compound with a metal halide. Theformation of the Grignard reagent and its subsequent reaction with ametal halide to form the precursor are carried out in an oxygencontaining solvent, typically an ether. Subsequent purificationprocesses are then performed to remove the oxygen containing ethersolvent and other impurities from the metalorganic precursor.

Unfortunately residual trace amounts of ether can result in oxygencontamination of semi-conductor structures grown using the aboveprepared precursors. Consequently, there is a deleterious effect on theproperties of the semiconductor structures.

The existence of metalorganic amine adducts, has been disclosed in forexample the reports of Henrickson C. H. et al (Inorganic Chemistry, vol.7, no 6 1968 pages 1047-1051) and Stevens, L. G. et al (Journal ofInorganic and Nuclear Chemistry, vol. 26, 1964, pages 97 -102).

An object of this invention is to provide a method of preparingmetalorganic compounds that avoids the above-mentioned disadvantages.

According to this invention there is provided a process for preparing ametalorganic compound by reacting a Grignard reagent with the metalhalide, characterised in that said reaction is carried out in an aminesolvent.

The Grignard reagent for use in the process of the invention ispreferably prepared in an amine solvent, especially the amine to be usedin preparing the metalorganic compound.

The amine is preferably a tertiary amine such as, for example, atertiary alkyl amine or a tertiary heterocyclic amine. Amines for use inthe invention are preferably liquid at room temperature, typically 18°to 20° C. Tertiary alkyl amines for use in the invention preferably havethe formula ##STR1## wherein R¹, R² and R³ are alkyl groups having from1 to 4 carbon atoms and wherein R¹, R², and R³ may be the same or two ofR¹, R² and R³ may be the same. Preferred alkyl amines for use in theinvention are triethylamine and dimethylethylamine.

Suitable heterocylic amines for use in the invention may includepyridine, 2H-pyrrole, pyrimidine, pyrazine, pyridazine, 1,3,5-triazineand hexahydrotriazine.

The Grignard reagent may be prepared in any suitable way, typically byreaction of magnesium with an alkyl halide, wherein the alkyl group isthat required for the metalorganic compound.

Metalorganic compounds that may be prepared in accordance with theinvention include alkyl compounds of Group II, Group III and Group Vmetals. Examples of such compounds include dialkyl zinc, dialkylcadmium, trialkyl aluminium, trialkyl gallium, trialkyl indium, trialkylphosphorus, trialkyl arsenic and trialkyl antimony.

It is believed that the process of the present invention results in anadduct of the metalorganic compound with the amine. The formation ofthis adduct permits the removal of volatile metallic and nonmetallicmicroimpurities from the metalorganic compound. Impurities may bereadily removed from the adduct by distillation. The adduct may be splitby removal of the amine, such as by heating, to provide the metalorganiccompound alone for some purposes, such as a precursor for MOVPE or CBE.Alternatively the adduct itself may be used as a precursor for thedeposition of, for example Group III-V or II-VI layers, such as galliumarsenide, aluminium gallium arsenide and zinc selenide, by MOVPE, CBEand other vapour phase epitaxy techniques.

A preferred process according to the invention includes the followingsteps:

1. Synthesis of RMgX in NR₃ solvent;

2. Suspension of MCl₃ in pentane;

3. Addition of RMgX to MCl₃ in NR₃ /pentane;

4. Removal of volatiles and isolation of MR₃ (NR₃) by distillation;

5. Removal of volatile impurities from MR₃ (NR₃);

6. Isolation of the adduct or thermal dissociation of MR₃ (NR₃) andremoval by fractional distillation of the NR₃ ligand.

The invention will now be further described by means of the followingexamples. Each reaction described below was carried out in an atmosphereof dry/oxygen-free dinitrogen using reagents which had been dried anddeoxygenated by standard purification methods.

EXAMPLE 1

This example demonstrates the production of triisopropylgallium usingtriethylamine as solvent.

A solution of iso-propyl magnesium bromide, i-PrMgBr, in triethylaminewas prepared by the dropwise addition of iso-propyl bromide, i-PrBr (280g, 2.3 mol) to a stirred suspension of magnesium metal turnings (60 g,2.5 mol) in triethylamine, NEt₃ (1000 cm³). This resulted in a vigorousexothermic reaction. It was found that this reaction could be moreeasily initiated by the addition of a crystal of iodine. After completeaddition of the i-PrBr, the reaction mixture was stirred at ambienttemperature for 4 hours.

A solution of gallium trichloride, GaCl₃ (125 g, 0.7 mol) in pentane(500 cm³) was then added slowly with stirring to the solution ofi-PrMgBr in NEt₃. This led to an exothermic reaction. After completeaddition of the GaCl₃ -pentane solution, the reaction mixture wasstirred for 4 hours at room temperature to ensure complete reaction.

After removal of volatiles by distillation in vacuo, the crude productwas isolated by vacuum distillation (100° C.) into a receiver cooled inliquid nitrogen (ca-196° C.). Volatile impurities were removed from thecrude product by distillation invacuo (25°-50° C.) and the pure liquidproduct was obtained by vacuum distillation (80° C.) into a cooledreceiver (ca-106° C.).

The metalorganic product was identified using proton NMR spectroscopy asa triethylamine adduct of triisopropylgallium, i-Pr₃ Ga(NEt₃)0.6·

The proton NMR data are summarised below:

    ______________________________________    (ppm)                (Assignment)    ______________________________________    0.8 (triplet, 5.4H)  NCH.sub.2 CH.sub.3    1.0 (multiplet, 3H)  GaCH(CH.sub.3).sub.2    1.4 (doublet, 18H)   GaCH(CH.sub.3).sub.2    2.4 (quartet, 3.6H)  NCH.sub.2 CH.sub.3    ______________________________________

The i-Pr₃ Ga-NEt₃ adduct was further analysed for trace metal impuritiesusing inductively coupled plasma emission spectroscopy (ICP-ES). Theonly impurities detected were silicon (0.03 ppm w.r.t. Ga) and. zinc(0.2ppm w.r.t. Ga).

Yield i-Pr₃ Ga(NEt₃)₀.6 =49.4 g.

The vapour pressure of the iPr₃ Ga adduct was found to be 0.9 mBar ar13° C.

The tri-isopropyl gallium prepared in the above way was used to grow alayer of AlGaAs on a gallium arsenide substrate by chemical beam epitaxyunder the following conditions:

    ______________________________________    Substrate temperature                      540° C.    AlGaAs growth rate                      1/hr    Group V precursor -                      thermally cracked arsine    Group III precursors -                      tri-isopropyl gallium                      triethylamine adduct plus                      AlH.sub.3 --NMe.sub.2 Et    ______________________________________

An AlGaAs layer (aluminium composition of 18%) grown in this mannerdemonstrated oxygen levels of less than 4×10¹⁶ cm⁻³ (as measured bysecondary ion mass spectrometry, SIMS). This layer is superior to anAlGaAs layer (aluminium composition of 25%) grown usingtriisopropylgallium synthesised in a conventional manner (i.e. using anether solvent), and AlH₃ (NMe₂ Et), in which much higher oxygen levelsof 9×10¹⁶ cm⁻³ were detected by SIMS. The AlGaAs layer grown using thetriisopropyl gallium-triethylamine adduct was comparable in oxygencontent (<4×10¹⁶ cm⁻³) with the best layers thus far obtained usingtriethylgallium and AlH₃ (NMe₂ Et) under identical CBE growthconditions.

FIGS. 1 and 2 respectively of the accompanying drawings show comparisonof vapour pressures and growth rates of the tri-isopropyl gallium adductprepared according to this Example and tri-isopropyl gallium prepared inthe conventional way. As can be seen the adduct has both higher vapourpressures and growth rates which are advantageous for chemical vapourdeposition processes.

EXAMPLE 2

This demonstrates the production of triisopropylgallium usingdimethylethylamine as solvent.

A solution of iso-propylmagnesium bromide, i-PrMgBr, indimethylethylamine was prepared by the dropwise addition ofiso-propylbromide, i-PrBr (166 g, 1.4 mol) to a stirred suspension of Mgmetal turnings (48 g, 2.0 mol) in dimethylethylamine, NMe₂ Et (500 cm³).This resulted in a vigorous exothermic reaction which could be moreeasily initiated by the addition of a small quantity of iodine. Aftercomplete addition of the i-PrBr the reaction mixture was stirred at roomtemperature for 4 hours.

A solution of GaCl₃ (69 g, 0.4 mol) in pentane (260 cm³) was then addedslowly, with stirring, to the solution of i-PrMgBr in NMe₂ Et. This ledto a vigorous exothermic reaction. After complete addition of the GaCl₃-pentane solution, the reaction mixture was stirred for 4 hours at roomtemperature to ensure complete reaction.

After removal of volatiles by atmospheric pressure distillation (60°C.), the crude product was isolated by vacuum distillation (100° C.)into a cooled receiver (ca-196° C.). Volatile impurities were removedfrom the crude products in vacuo, and the pure liquid product wasobtained by reduced pressure distillation (70° C.) into a receiver.

The metalorganic product was identified using proton NMR spectroscopy asthe dimethylethylamine adduct of triisopropylgallium, i-Pr₃ Ga(NMe₂ Et).The proton NMR data are summarised below:

    ______________________________________    (ppm)                (Assignment)    ______________________________________    0.6 (triplet, 3H)    NCH.sub.2 CH.sub.3    0.9 (multiplet, 3H)  GaCH(CH.sub.3).sub.2    1.4 (doublet, 18H)   GaCH(CH.sub.3).sub.2    1.9 (singlet, 6H)    NCH.sub.3    2.4 (quartet, 2H)    NCH.sub.2 CH.sub.3    ______________________________________

The i-Pr₃ Ga-NMe₂ Et adduct was further analysed for trace metalimpurities using ICP-ES. The only impurities detected were silicon (0.2ppm w.r.t Ga) and Zinc (4.6 ppm w.r.t Ga).

Yield i-Pr₃ Ga(NMe₂ Et)=58.5 g

EXAMPLE 3

This example demonstrates the production of triisopropylindium usingtriethylamine as solvent.

A solution of i-PrMgBr in NEt₃ was prepared by the dropwise addition ofi-PrBr (72 g, 0.6mol) in NEt₃ (200 cm³). This led to a vigorousexothermic reaction. After complete addition of the i-PrBr the reactionmixture was stirred at room temperature for 4 hours.

The solution of i-PrMgBr in NEt₃ was added dropwise, with stirring, to asuspension of indium trichloride, InCl₃ (35 g, 0.2 mol) in NEt₃ (200cm³). This led to an exothermic reaction. After complete addition of thei-PrMgBr/NEt₃ solution, the reaction mixture was boiled under reflux for2 hours.

After removal of volatiles by distillation in vacuo, the crude productwas obtained by vacuum distillation (100° C.) into a cooled receiver(ca-196° C). Volatile impurities were removed from the crude product bydistillation in vacuo and the pure liquid product was obtained by vacuumdistillation (70° C.) into a cooled receiver (ca-196° C.).

The metalorganic product was identified using proton NMR spectroscopy asa triethylamine adduct of triisopropylindium, i-Pr₃ In(NEt₃). The protonNMR data are summarised below:

    ______________________________________    (ppm)                (Assignment)    ______________________________________    0.8 (triplet, 9H)    NCH.sub.2 CH.sub.3    1.1 (multiplet, 3H)  InCH(CH.sub.3).sub.2    1.6 (doublet, 18H)   InCH(CH.sub.3).sub.2    2.4 (quartet, 6H)    NCH.sub.2 CH.sub.3    ______________________________________

The i-Pr₃ In-NEt₃ adduct was further analysed for trace metal impuritiesusing ICP-ES. The only impurities detected were silicon (0.04 ppm w.r.tIn) and zinc (3.8 ppm w.r.t In).

Yield i-Pr₃ In(NEt₃)=8 g.

EXAMPLE 4

This example demonstrates the production of triisopropylindium usingdimethylethylamine as solvent.

A solution of i-PrMgBr in NMe₂ Et was prepared by the dropwise additionof i-PrBr (192 g, 1.6 mol) to a stirred suspension of Mg metal turnings(56 g, 2.3 mol) in NMe₂ Et (400cm³).

This resulted in a vigorous exothermic reaction. After complete additionof the i-PrBr the reaction mixture was stirred for 4 hours at roomtemperature.

The solution of i-PrMgBr in NMe₂ Et was added dropwise, with stirring,to a suspension of InCl₃ (72 g, 0.3 mol) in pentane. This resulted in anexothermic reaction. After complete addition of the i-PrMgBr/NMe₂ Etsolution, the reaction mixture was boiled under reflux for 2 hours.

After removal of volatiles by atmospheric pressure distillation, (60°C.), the crude product was obtained by reduced pressure distillation(85°-90° C.) into a receiver. Volatile impurities were removed from thecrude product by vacuum distillation (25° C.).

The pure liquid product was obtained by vacuum distillation (85°-90° C.)into a receiver cooled to approx. -196° C.

The straw yellow liquid was identified using proton NMR spectroscopy asthe dimethylethylamine adduct of tri-isopropyl indium, iPr₃ In(NMe₂ Et).The proton NMR data are summarised below:

    ______________________________________    (ppm)                (Assignment)    ______________________________________    0.8 (triplet, 3H)    NCH.sub.2 CH.sub.3    1.0 (multiplet, 3H)  InCH(CH.sub.3).sub.2    1.5 (doublet, 18H)   InCH(CH.sub.3).sub.2    2.0 (singlet, 6H)    NCH.sub.3    2.3 (quartet, 2H)    NCH.sub.2 CH.sub.3    ______________________________________

The i-Pr₃ In-NMe₂ Et adduct was further analysed for trace metalimpurities using ICP-EAS. The only impurities detected were silicon (<1ppm) w.r.t In), and Zn(0.12 w.r.t In).

Yield i-Pr₃ In(NMe₂ Et)=81.7 g.

We claim:
 1. A process for preparing a Group II, Group III or Group Vmetalorganic compound comprising reacting in a tertiary alkyl aminesolvent a Grignard reagent with a Group II, Group III or Group V metalhalide to form a metalorganic adduct, isolating the adduct anddissociating the adduct to leave the metalorganic compound.
 2. A processas claimed in claim 1, wherein the amine is liquid at room temperature.3. A process as claimed in claim 1, wherein the amine has the formula:##STR2## wherein R¹, R² and R³ are alkyl groups having from 1 to 4carbon atoms and wherein R¹, R² and R³ are the same or two of R¹, R² andR³ are the same.
 4. A process as claimed in claim 3, wherein the amineis selected from the group consisting of triethylamine anddimethylethylamine.
 5. A process as claimed in claim 1, wherein themetalorganic compound is selected from the group consisting of trialkylaluminium, trialkyl gallium, trialkyl indium, trialkyl phosphorous,trialkyl arsenic and trialkyl antimony.
 6. A process as claimed in claim5, wherein the alkyl groups of the metalorganic compound compriseisopropyl groups.
 7. A process as claimed in claim 5, wherein the alkylgroups of the metalorganic compounds include one or more isopropylgroups.
 8. A process for preparing a Group II, Group III or Group Vmetalorganic compound comprising the steps of preparing a Grignardreagent in a tertiary alkyl amine solvent; reacting the Grignard reagentwith a Group II, Group III or Group V metal halide in the tertiary alkylamine solvent to form a metalorganic adduct; isolating the metalorganicadduct formed; and dissociating the metalorganic adduct to leave themetalorganic compound.
 9. A process as claimed in claim 8, wherein theGrignard reagent is prepared by reacting magnesium with an alkyl halideand wherein the metalorganic compound contains an alkyl group suppliedby the metal halide.